cpp is quite close tied to its compiler, but it would probably be possible to write a sh wrapper for cpp to adapt to another one.
You may look at the -B option to make it search for cpp on alternate places.

replace /usr/libexec/cpp with the script, so when pcc front end calls the 'preprocessor' the script will build a different but equivalent command line for GCC cpp and call that program instead. I did not test it beyond superficially by the way and I expect there will be cases it does not handle correctly but you should be able to see how it works more or less if you want to extend it..